Xinhua News Agency said Sunday that a loose drainage valve in a chemical plant was to blame for the leaking of nearly 9 tons of the chemical aniline into the river. It said another 30 tons of aniline have been contained in a disused reservoir.

Ma Jun, director of the nonprofit Institute of Public & Environmental Affairs in Beijing, described the incident in an interview as “serious” due to both the amount of pollutants involved and the toxicity of the substance. But he said the broader problem was the delay in making it public. “The government should do a thorough investigation,” Mr. Ma said.

Changzhi, the upriver city in Shanxi province where the initial accident occurred, illustrates the challenges China faces in protecting its natural resources. In a marketing video, historic Changzhi bills itself as the nation’s “attractive city” in the cradle of Chinese civilization, with plentiful clean water that runs through a rugged mountain range with Grand Canyon-like vistas. Yet Changzhi in the video also boasts an industrial backbone of coal mining, chemical production and steel manufacturing.

Users of Sina Corp.’s SINA -0.02% Twitter-like Weibo microblog service posted photos from Handan of store shelves emptied of bottled water and criticized the time it took for authorities to report the leak. Chinese bloggers also reposted a report from the national broadcaster China Central Television that included a photo of the Tianji plant’s ruptured pipeline.

Water pollution is a serious problem in China, with garbage blamed for clogging dams, refineries for damaging marine life and fertilizers for ruining aquifers. Acute shortages of water through much of the country have been worsened by industrial accidents and often slow reporting by polluters. Still, it isn’t clear whether the Changzhi incident shows a worsening of the situation or greater public attention to it.